Proteins Flashcards
What is the general structure of an amino acid?
- COOH carboxyl group
- R variable side group consists of carbon chain & may include other functional groups
- NH2 amine group
Describe how to test for proteins in a sample
- Add equal volume of NaOH to sample at room temperature
- Add drop of dilute CuSO4 solution. Swirl to mix = biuret reagent
- Positive: colour changes from blue to purple
Negative: solution remains blue
How many amino acids are there and how do they differ from one another?
20
differ only by side ‘R’ group
How do dipeptides and polypeptides form?
- condensation reaction forms peptide bond (-CONH-) & eliminates molecule of water
How many levels of protein structure are there?
4
Define ‘primary structure’ of a protein
- sequence, number & type of amino acids in the polypeptide
- determined by sequence of codons on mRNA
Define ‘secondary structure’ of a protein
Hydrogen bonds form between Oδ- attached to -C=O & Hδ+ attached to -NH
Describe the 2 types of secondary protein structure
α-helix
- all N-H bonds on the same side of protein chain
- spiral shape
- H-bonds parallel to helical axis
β-pleated sheet
- N-H & C=O groups alternate from one side to the other
Define ‘tertiary structure’ of a protein & name the bonds
3D structure formed by further folding of polypeptide
- disulfide bridges
- ionic bonds
- hydrogen bonds
Describe disulfide bridges
Strong covalent S-S bonds between molecules of the amino acid cysteine
Describe ionic bonds
Relatively strong bonds between charged R groups (pH changes cause these bonds to break)
Describe hydrogen bonds
Numerous & easily broken
Define ‘quaternary structure’ of a protein
- Functional proteins may consist of more than one polypeptide
- Precise 3D structure held together by the same types of bond as tertiary structure
- May involve addition of prosthetic groups
Describe the structure and function of globular proteins
- spherical & compact
- hydrophilic R groups face outwards & hydrophobic R groups face inwards = usually water-soluble
- involved in metabolic processes
Describe the structure and function of fibrous proteins
- can form long chains or fibres
- insoluble in water
- useful for structure and support e.g collagen in skin
Outline how chromatography could be used to identify the amino acids in a mixture
- Use capillary tube to spot mixture onto pencil origin line & place chromatography paper in solvent
- Allow solvent to run until it almost touches other end of paper. Amino acids move different distances based on relative attraction to paper & solubility in solvent
- Use revealing agent or UV light to see spots
- Calculate Rf values & match to database
What are enzymes?
- Biological catalysts for intra & extracellular reactions
- Specific tertiary structure determines shape of active site, complementary to a specific substrate
- Formation of ES complexes lowers activation energy of metabolic reactions
Explain the induced fit model of enzyme action
- Shape of active site is not directly complementary to substrate & is flexible
- Conformational change enables ES complexes to form
- This puts strain on substrate bonds, lowering activation energy
How have models of enzyme action changed?
- Initially lock & key model: rigid shape of active site complementary to only 1 substrate
- Currently induced fit model: also explains why binding at allosteric sites can change shape of active site
How could a student identify the activation energy of a metabolic reaction from an energy level diagram?
Difference between free energy of substrate & peak of curve
Name 5 factors that affect the rate of enzyme-controlled reactions
- enzyme concentration
- substrate concentration
- concentration of inhibitors
- pH
- temperature
How does substrate concentration affect rate of reaction?
Given that enzyme concentration is fixed, rate increases proportionally to substrate concentration
Rate levels off when maximum number of ES complexes form at any given time
How does enzyme concentration affect rate of reaction?
Given that substrate is in excess rate increases proportionally to enzyme concentration
Rate levels off when maximum number of ES complexes form at any given time
How does temperature affect rate of reaction?
Rate increases as kinetic energy increases & peaks at optimum temperature
Above optimum, ionic & H-bonds in tertiary structure break = active site no longer complementary to substrate - denatured
